(a) Field of the Invention
The present invention relates to nickel fine powder, which is surface-modified with barium titanate having a perovskite structure, nickel fine powder, which is surface-modified with a precursor of the barium titanate and a method for preparing the surface-modified nickel fine powder. More specifically, the present invention relates to nickel fine powder surface-modified with barium titanate having a perovskite structure and nickel fine powder surface-modified with a precursor of the barium titanate, which have low heat shrinkage factors upon firing and improved resistance to oxidation during firing, which permit the inhibition of any such phenomenon that nickel is oxidized and the oxidized nickel diffuses into a ceramic dielectric material during firing, which is suitable for use as a material for a conductive paste and, in particular, for forming a conductive paste used in making a multilayer ceramic capacitor, as well as a method for preparing the same.
(b) Description of the Prior Art
Conventionally, there have mainly been used noble metal pastes as conductive pastes. However, there have recently been used base metal pastes in order to reduce the production cost and, in particular, nickel pastes have widely been used.
However, the nickel paste suffers from the following various problems. For instance, the nickel paste has a high heat shrinkage factor upon firing as compared with the noble metal paste. In addition, it is inferior in the resistance to oxidation and as a result, there has been observed a phenomenon in which nickel is oxidized and the resulting nickel oxide diffuses into a ceramic dielectric material during firing.
To solve the foregoing drawbacks observed when a nickel paste is used, various techniques have been reported and proposed. For instance, Japanese Un-Examined Patent Publication No. Sho 57-30308 discloses a method comprising the step of adsorbing, on the surface of nickel fine particles, ceramic powder (co-material) having a composition identical to that of a dielectric ceramic material, which constitutes a ceramic capacitor. In addition, Japanese Un-Examined Patent Publication No. Hei 11-124602 discloses a method comprising the step of forming, on the surface of nickel fine particles, a layer of a double oxide represented by the following general formula:
Ax By O(x+2y) 
(In the general formula, A represents at least one element selected from the group consisting of Ca, Sr and Ba; B represents at least one element selected from the group consisting of Ti and Zr; x and y are numerical values which satisfy the relation: 0.5xe2x89xa6y/xxe2x89xa64.5).
If a dense barium titanate (BaTiO3) film can be closely adhered to the surface of nickel fine particles, the resulting nickel fine particles permits the formation of a conductive nickel paste, which has a small heat shrinkage factor upon firing, improved resistance to oxidation and which can inhibit any occurrence of a phenomenon such that nickel is oxidized and the resulting nickel oxide diffuses into a ceramic dielectric material during firing. In fact, however, barium titanate has low adhesion to the surface of nickel fine particles and thus it is quite difficult to form a dense barium titanate film on the surface of nickel fine particles.
For instance, in the method disclosed in Japanese Un-Examined Patent Publication No. Sho 57-30308, the dielectric ceramic powder (co-material) and the nickel fine powder do not exert any adhesive force against each other and therefore, they are easily disconnected from one another upon the preparation of a paste. For this reason, the resulting paste is not sufficiently improved in the heat shrinkage factor upon firing, the resistance to oxidation and the phenomenon such that the oxidized nickel diffuses into the ceramic dielectric material during firing. Moreover, in the method disclosed in Japanese Un-Examined Patent Publication No. Hei 11-124602, the nickel powder is formed by spraying a solution containing respective pyrolytic compounds capable of forming a double oxide and a nickel raw material and then thermally decomposing these ingredients to form nickel powder containing the double oxide thus pyrolytically formed. However, this method results in the formation of the double oxide within the nickel particle.
Accordingly, it is an object of the present invention to provide nickel fine powder, which is surface-modified with barium titanate having a perovskite structure, which has a low heat shrinkage factor upon firing and improved resistance to oxidation, and which permit the inhibition of any such phenomenon that nickel is oxidized and the oxidized nickel penetrate into a ceramic dielectric material during firing.
It is another object of the present invention to provide nickel fine powder, which is surface-modified with barium titanate having a perovskite structure, which is suitable for use as a material for a conductive paste and, in particular, for forming a conductive paste used in making a multilayer ceramic capacitor.
It is a further object of the present invention to provide nickel fine powder, which is surface-modified with a precursor of the barium titanate.
It is a still further object of the present invention to provide a method for preparing the foregoing nickel fine powder, which is surface-modified with barium titanate having a perovskite structure and with a precursor of the barium titanate.
The inventors of this invention have conducted various studies to solve the foregoing problems associated with the conventional nickel fine powder used for making a conductive nickel paste. As a result, the inventors have found that the foregoing objects of the present invention can efficiently be achieved by bringing nickel fine particles into contact with a solution containing a combination of a soluble titanium compound and a soluble barium compound, which can form barium titanate having a perovskite structure through a heat-treatment, to thus form a precursor consisting of a reaction product of the soluble titanium compound and the soluble barium compound and to thus adhere the precursor to the surface of the individual nickel fine particles, then drying the nickel fine particles provided thereon with the precursor attached thereto (or adhering the precursor to the surface of the individual nickel fine particles, drying them and subjecting them to a heat-treatment at a temperature of less than 400xc2x0 C.) (at this stage, a precursor consisting of the reaction product of the soluble titanium-containing compound and the soluble barium-containing compound is formed on the surface of the nickel fine particles), then subjecting them to a heat-treatment at a temperature of not less than 400xc2x0 C. to thus convert the precursor on the surface of the nickel fine powder into barium titanate having a perovskite structure and to thus cover the surface of the nickel fine particles with the barium titanate having a specific structure, in place of directly covering the surface of nickel fine particles with barium titanate having a perovskite structure. The inventors have thus completed the present invention on the basis of the foregoing finding.
According to an aspect of the present invention, there is provided surface-modified nickel fine powder. The surface-modified nickel fine powder of the present invention is nickel fine powder, in which a precursor of barium titanate having a perovskite structure and consisting of a reaction product of a soluble titanium-containing compound and a soluble barium-containing compound is attached to the surface of individual nickel fine particles. The surface-modified nickel fine powder is characterized in that the fine powder shows X-ray diffraction peaks of nickel and is free of any peak of barium titanate having a perovskite structure in the X-ray diffraction pattern determined by the X-ray diffraction method, but the fine powder shows X-ray diffraction peaks of both nickel and barium titanate having a perovskite structure in the X-ray diffraction pattern after the fine powder is subjected to a heat-treatment at a temperature of not less than 400xc2x0 C.
According to a second aspect of the present invention, there is provided a method for preparing nickel fine powder surface-modified with a precursor. The method of the present invention comprises the steps of bringing nickel fine particles into contact with a solution containing a combination of a soluble titanium-containing compound and a soluble barium-containing compound, which can form barium titanate having a perovskite structure through a heat-treatment at a temperature of not less than 400xc2x0 C., to thus adhere, to the surface of the individual nickel fine particles, a precursor consisting of the reaction product of the soluble titanium-containing compound and the soluble barium-containing compound and then drying the fine particles; or adhering such a precursor to the surface of the individual nickel fine particles, then drying the same and heat-treating the particles at a temperature of less than 400xc2x0 C.
According to a third aspect of the present invention, there is provided nickel fine powder surface-modified with barium titanate having a perovskite structure, in which the barium titanate having a perovskite structure is adhered to the surface of the individual nickel fine particles and the rate of increase in the weight of the fine powder observed after allowing them to stand in the atmosphere maintained at 400xc2x0 C. for 2 hours is not more than xc2xd time that observed for the untreated elemental nickel fine powder.
According to a fourth aspect of the present invention, there is provided a method for preparing the foregoing nickel fine powder surface-modified with the barium titanate having a perovskite structure. The method comprises the steps of bringing nickel fine particles into contact with a solution containing a combination of a soluble titanium-containing compound and a soluble barium-containing compound, which can form barium titanate having a perovskite structure through a heat-treatment at a temperature of not less than 400xc2x0 C., to thus adhere, to the surface of the individual nickel fine particles, a precursor consisting of the reaction product of the soluble titanium-containing compound and the soluble barium-containing compound, then drying the fine particles and heat-treating the fine particles at a temperature of not less than 400xc2x0 C.; or adhering such a precursor to the surface of the individual nickel fine particles, then drying the same, heat-treating the particles at a temperature of less than 400xc2x0 C. and thereafter further heat-treating the fine particles at a temperature of not less than 400xc2x0 C.